Genetic instability is associated with increased rates of mutation, chromosome rearrangement, and aneuploidy. This is a hallmark of cancer, as well as other significant health challenges including developmental defects, neurological disorders, and aging. Data from multiple systems suggests that DNA replication stress is a key contributor to genome instability. Molecular mechanisms that stabilize replication forks, prevent abnormal divisions, and promote DNA repair are a primary barrier to disease; therefore, understanding their function has direct relevance to human health. This proposal employs an established model genetic system to identify and characterize molecular pathways that maintain genome stability. The project uses genetics, molecular biology, and novel cell biology methods in the fission yeast S. pombe to examine the response of cells to replication stress during the normal vegetative cell cycle, and during meiosis. S. pombe is a well-established model for chromosome biology that shares many features with human cells. The proposal investigates the hypothesis that the dynamics of the response to replication stress determines whether cells arrest the cell cycle, or whether they evade normal checkpoints and go on to divide abnormally, generating chromosome rearrangements and increased rates of mutation. Replication stress may vary across the genome and a significant component of the study is the analysis of the pericentromere as a model fragile site. An additional novel component is the analysis of replication stress during meiosis as a contributor to chromosome rearrangements associated with birth defects and infertility. A significant innovation is a new system for live cell pedigree analysis coupled with quantitative analysis to investigate the dynamic response to damage and checkpoint evasion. This live whole-cell analysis allows the identification of distinct sub-populations of cells that undergo different outcomes creating a cycle of instability that is associated with multiple diseases By combining this new cell biological approach with superb yeast gene-discovery tools, and identifying the molecular events that lead to abnormal divisions and further stress, this project will tackle a critical gap in current understanding. What are the pathways that contribute to different responses to stress and their associated pathologies? How do they differ from one another to generate distinct outcomes such as clustered mutations, CNVs, deletions and duplications, and chromosome rearrangements? Together, these studies provide a holistic picture of how conserved proteins interact to maintain genome stability in a eukaryotic cell, identifying markers and risk factors for human disease.